Space-time processing for wireless systems with multiple transmit and receive antennas

ABSTRACT

Signals are developed for use in a wireless system with multiple transmit and multiple receive antennas so that even in the face of some correlation the most open-loop capacity that can be achieved using a substantially open-loop system with a channel of that level of correlation is obtained. In accordance with the principles of the invention, the signals transmitted from the various antennas are processed so as to improve their ability to convey the maximum amount of information. More specifically, the data to be transmitted is divided into M+1 substreams, where M is the number of transmit antennas. Each transmit antenna is supplied with a combination signal that is made up of a weighted version of a common one of the substreams and a weighted version of a respective one of the substreams that is supplied uniquely for that antenna, so that there are M transmit signals. A receiver having N antennas receives the M transmit signals as combined by the channel and reconstitutes the original data therefrom. This may be achieved using successive decoding techniques. Advantageously, the capacity, i.e., the rate of information that can be conveyed with an arbitrarily small probability of error when the instantaneous forward channel condition is unknown to the transmitter, is maximized.

This application is a division of application Ser. No. 09/641,414, filedAug. 18, 2000, now U.S. Pat. No. 6,778,612.

TECHNICAL FIELD

This invention relates to the art of wireless communications, and moreparticularly, to wireless communication systems using multiple antennasat the transmitter and multiple antennas at the receiver, so calledmultiple-input multiple-output (MIMO) systems.

BACKGROUND OF THE INVENTION

It is well known in the art that multiple-input multiple-output (MIMO)systems can achieve dramatically improved capacity as compared to singleantenna, i.e., single antenna to single antenna or multiple antenna tosingle antenna, systems. However, to achieve this improvement, it ispreferable that there be a rich scattering environment, so that thevarious signals reaching the multiple receive antennas be largelyuncorrelated. If the signals have some degree of correlation, and suchcorrelation is ignored, performance degrades and capacity is reduced.

SUMMARY OF THE INVENTION

We have invented a way of developing signals in a MIMO system such thateven in the face of some correlation the most open-loop capacity thatcan be achieved using a channel of that level of correlation isobtained. In accordance with the principles of the invention, thesignals transmitted from the various antennas are processed so as toimprove their ability to convey the maximum amount of information. Morespecifically, the data to be transmitted is divided into M+1 substreams,where M is the number of transmit antennas. Each transmit antenna issupplied with a combination signal that is made up of a weighted versionof a common one of the substreams and a weighted version of a respectiveone of the substreams that is supplied uniquely for that antenna, sothat there are M transmit signals. A receiver having N antennas receivesthe M transmit signals as combined by the channel and reconstitutes theoriginal data therefrom. This may be achieved using successive decodingtechniques. Advantageously, the open-loop capacity, i.e., the rate ofinformation that can be conveyed with an arbitrarily small probabilityof error when the instantaneous forward channel condition is unknown tothe transmitter, is maximized.

In one embodiment of the invention, the weights are determined by theforward channel transmitter using channel statistics of the forward linkwhich are made known to the transmitter of the forward link by beingtransmitted from time to time from the receiver of the forward link bythe transmitter of the reverse link. In another embodiment of theinvention, a determination of weight parameter, or the weightsthemselves, is made by the forward channel receiver using the channelstatistics of the forward link and the determined weight parameter, orweights, is made known to the transmitter of the forward link by beingtransmitted from time to time from the receiver of the forward link bythe transmitter of the reverse link.

BRIEF DESCRIPTION OF THE DRAWING

In the drawing:

FIG. 1 shows an exemplary portion of a transmitter for developingsignals to transmit in a MIMO system having a transmitter with Mtransmit antennas transmitting over a forward channel, such that even inthe face of some correlation the most open-loop capacity that can beachieved with a channel of that level of correlation is obtained, inaccordance with the principles of the invention;

FIG. 2 shows an exemplary portion of a receiver for a MIMO systemarranged in accordance with the principles of the invention;

FIG. 3 shows an exemplary process, in flow chart form, for developingsignals to transmit in a MIMO system such that even in the face of somecorrelation the most open-loop capacity that can be achieved with achannel of that level of correlation is obtained with a substantiallyopen-loop process, in accordance with the principles of the invention;

FIG. 4 shows another exemplary process, in flow chart form, fordeveloping signals to transmit in a MIMO system such that even in theface of some correlation the most open-loop capacity that can beachieved with a channel of that level of correlation is obtained with asubstantially open-loop process, in accordance with the principles ofthe invention; and

FIG. 5 shows an another exemplary portion of a transmitter fordeveloping signals to transmit in a MIMO system having a transmitterwith M transmit antennas transmitting over a forward channel, such thateven in the face of some correlation the most open-loop capacity thatcan be achieved with a channel of that level of correlation is obtained,in accordance with the principles of the invention.

DETAILED DESCRIPTION

The following merely illustrates the principles of the invention. Itwill thus be appreciated that those skilled in the art will be able todevise various arrangements which, although not explicitly described orshown herein, embody the principles of the invention and are includedwithin its spirit and scope. Furthermore, all examples and conditionallanguage recited herein are principally intended expressly to be onlyfor pedagogical purposes to aid the reader in understanding theprinciples of the invention and the concepts contributed by theinventor(s) to furthering the art, and are to be construed as beingwithout limitation to such specifically recited examples and conditions.Moreover, all statements herein reciting principles, aspects, andembodiments of the invention, as well as specific examples thereof, areintended to encompass both structural and functional equivalentsthereof. Additionally, it is intended that such equivalents include bothcurrently known equivalents as well as equivalents developed in thefuture, i.e., any elements developed that perform the same function,regardless of structure.

Thus, for example, it will be appreciated by those skilled in the artthat the block diagrams herein represent conceptual views ofillustrative circuitry embodying the principles of the invention.Similarly, it will be appreciated that any flow charts, flow diagrams,state transition diagrams, pseudocode, and the like represent variousprocesses which may be substantially represented in computer readablemedium and so executed by a computer or processor, whether or not suchcomputer or processor is explicitly shown.

The functions of the various elements shown in the FIGS., includingfunctional blocks labeled as “processors” may be provided through theuse of dedicated hardware as well as hardware capable of executingsoftware in association with appropriate software. When provided by aprocessor, the functions may be provided by a single dedicatedprocessor, by a single shared processor, or by a plurality of individualprocessors, some of which may be shared. Moreover, explicit use of theterm “processor” or “controller” should not be construed to referexclusively to hardware capable of executing software, and mayimplicitly include, without limitation, digital signal processor (DSP)hardware, read-only memory (ROM) for storing software, random accessmemory (RAM), and non-volatile storage. Other hardware, conventionaland/or custom, may also be included. Similarly, any switches shown inthe FIGS. are conceptual only. Their function may be carried out throughthe operation of program logic, through dedicated logic, through theinteraction of program control and dedicated logic, or even manually,the particular technique being selectable by the implementor as morespecifically understood from the context.

In the claims hereof any element expressed as a means for performing aspecified function is intended to encompass any way of performing thatfunction including, for example, a) a combination of circuit elementswhich performs that function or b) software in any form, including,therefore, firmware, microcode or the like, combined with appropriatecircuitry for executing that software to perform the function. Theinvention as defined by such claims resides in the fact that thefunctionalities provided by the various recited means are combined andbrought together in the manner which the claims call for. Applicant thusregards any means which can provide those functionalities as equivalentas those shown herein.

FIG. 1 shows an exemplary portion of a transmitter for developingsignals to transmit in a MIMO system having a transmitter with Mtransmit antennas transmitting over a forward channel, such that even inthe face of some correlation the most open-loop capacity that can beachieved with a channel of that level of correlation is obtained, inaccordance with the principles of the invention. Shown in FIG. 1 are a)demultiplexer (demux) 101; b) weight supplier 105; c) antennas 107,including antennas 107-1 through 107-M; d) adders 109, including adders109-1 through 109-M; e) multipliers 111-1 through 111 -M+1; and f) radiofrequency (RF) converters 115; including 115-1 through 115-M.

Demultiplexer 101 takes a data stream as an input and supplies as anoutput M+1 data substreams by supplying various bits from the input datastream to each of the data substreams. The data substreams are suppliedby demultiplexer 101 to a respective one of multipliers 111. Multiplier111-1 through 111-M multiply each value of the first M data substreamsby a first weight supplied by weight supplier 105. Typically, each ofthe first M weighted data substreams are of equal rate. Similarly,multiplier 111-M+1 multiplies each value of the M+1^(th) data substreamby a second weight supplied by weight supplier 105.

Typically the M+1^(th) data substream is not at the same rate as thefirst M data substreams. As will be recognized by those of ordinaryskill in the art, the particular rates for the first M data substreamsand the M+1^(th) data substream are dependent on the receiver, inparticular, the order in which the receiver performs the successivedecomposition. Thus, the particular rates are typically negotiated fromtime to time between the receiver and the transmitter. Note that themore correlated the channel is, the larger the rate of the M+1^(th) datasubstream.

The first and second weights may be related to each other, and may bedeveloped by weight supplier 105 from a common weight parameter whichmay be derived from statistics of the forward channel, as will bedescribed in more detail hereinbelow. In one embodiment of theinvention, weight supplier 105 actually develops the weight values inresponse to information received via the reverse channel from thereceiver shown and described further in FIG. 2. In another embodiment ofthe invention the weight values are developed in the receiver, thensupplied via the reverse channel to the transmitter, in which they arestored in weight supplier 105 until such time as they are required. Aprocess for developing the weights in accordance with an aspect of theinvention will be described hereinbelow.

Each of the first M weighted data substreams is supplied as an input ofa respective one of adders 109. Each of adders 109 also receives at itsother input the weighted M+1^(th) data substream which is supplied as anoutput by multiplier 111-M+1. Each of adders 109 combines the twoweighted data substreams input to it so as to produce a combined branchsignal. Thus, M combined branch signal are produced, one by each ofadders 109. Each of radio frequency (RF) converters 115 receives one ofthe M combined branch signals and develops therefrom radio frequencyversions of the M combined branch signals, which are then supplied torespective ones of antennas 107 for transmission.

FIG. 2 shows an exemplary portion of a receiver for a MIMO systemarranged in accordance with the principles of the invention. FIG. 2shows a) N antennas 201, including antennas 201-1 through 201-N; b)radio frequency (RF) converters 203, including radio frequency (RF)converters 203-1 through 203-N; c) channel statistics estimation unit207; e) optional weight parameter calculator 209; and f) optional switch211. Each of antennas 201 receives radio signals and supplies anelectrical version thereof to its respective, associated one of radiofrequency (RF) converters 203. Each of radio frequency (RF) converters203 downconverts the signal it receives to baseband, converts thebaseband analog signal it received to a digital representation, andsupplies the digital representation to channel statistics estimationunit 207.

Channel statistics estimation unit 207 develops certain statisticsregarding the channel. In particular, channel statistics estimation unit207 may develop a) an estimate of the averagesignal-to-interference-and-noise ratio (SINR), ρ, and b) the correlationamong the channel components, η. The correlation among the channelcomponents is developed using an estimate of the forward matrix channelresponse which is developed in the conventional manner. Note thatmatrices are required because there are multiple transmit antennas andmultiple receive antennas. More specifically, the correlation among thechannel components over a time period may be computed as η=K/(K+1),where K is the well known Ricean spatial K-factor

The channel statistics are supplied either to optional weight parametercalculator 209 or they are supplied via the reverse channel to thetransmitter (FIG. 1). If the channel statistics are supplied to weightparameter calculator 209, weight parameter calculator 209 determines theweight parameter that is to be used, in accordance with an aspect of theinvention and as described hereinbelow, and supplies the resultingweight parameter to the transmitter (FIG. 1) via the reverse channel.

FIG. 3 shows an exemplary process, in flow chart form, for developingsignals to transmit in a MIMO system having a transmitter with Mtransmit antennas transmitting over a forward channel to a receiverhaving N receiver antennas and a reverse channel for communicating fromthe receiver to the transmitter, such that even in the face of somecorrelation the most open-loop capacity that can be achieved with achannel of that level of correlation is obtained with a substantiallyopen-loop process, in accordance with the principles of the invention.The process of FIG. 3 may be employed in an embodiment of the inventionthat uses the hardware of FIGS. 1 and 2, with switch 211 being connectedto channel statistics estimation unit 207 as follows.

First it is necessary to determine the length of time during which thechannel statistics are stable. This is typically performed at the systemengineering phase of developing the system, using measurements of theenvironment into which the system is to be deployed, as is well known bythose of ordinary skill in the art. Once the length of time for whichthe channel statistics are stable is known, that time is the time periodover which information will be gathered to generate each statistic.

The process of FIG. 3 is entered in step 301 at the beginning of eachtime period. Next, in step 303, the channel statistics are estimatedover the time period.

Thereafter, in step 305, (FIG. 3) the statistics are supplied by thereceiver of the forward link to the transmitter of forward link, e.g.,via the reverse channel.

In step 307 the first and second weights, α₁ and α₂ are calculated,e.g., by weight supplier 105 (FIG. 1). More specifically, the weightsare calculated as follows.

$\begin{matrix}{\alpha_{1} = \sqrt{\frac{P_{T}}{M} - \alpha_{2}^{2}}} \\{\alpha_{2} = \sqrt{\frac{P_{T}\eta}{\rho\;{N( {1 - \eta} )}( {{M\;\eta} + 1 - \eta} )}}}\end{matrix}$where M, N, ρ, η are as defined hereinabove and P_(T) is the totalavailable transmit power. Thus it can be seen that there is arelationship between the two weights, allowing one of them to act as theweight parameter from which the other is determined, e.g., according tothe following

M(α₁² + α₂²) = P_(T).

In step 309, the input data stream is divided into M+1 substreams e.g.,by demultiplexer 101 (FIG. 1). Each of the first M data substreams isthen multiplied by weight α₁ in step 311 (FIG. 3). In other words, eachbit of each particular data stream is multiplied by α₁ to produce Mweighted data substreams. Additionally, the M+1^(th) data substream ismultiplied by α₂ to produce the M+1^(th) weighted data substream.

In step 313, each of the first M weighted data substreams is combinedwith the M+1^(th) weighted data substream, e.g., by adders 109. Theprocess then exits in step 315.

FIG. 4 shows another exemplary process, in flow chart form, fordeveloping signals to transmit in a MIMO system having a transmitterwith M transmit antennas transmitting over a forward channel to areceiver having N receiver antennas and a reverse channel forcommunicating from the receiver to the transmitter, such that even inthe face of some correlation the most open-loop capacity that can beachieved with a channel of that level of correlation is obtained with asubstantially open-loop process, in accordance with the principles ofthe invention. The process of FIG. 4 may be employed in an embodiment ofthe invention that uses the hardware of FIGS. 1 and 2, with switch 211being connected to weight calculator 209. Note that for the process ofFIG. 4, weight supplier 105 of FIG. 1 will not compute the variousweights, but will instead merely store the weights received from weightcalculator 209 and supply them to the various ones of multipliers 113 asis necessary.

The process of FIG. 4 is entered in step 401 at the beginning of eachtime period. Next, in step 404, the channel statistics are estimatedover the time period.

In step 405 at least one of the weights α₁ and α₂ are calculated e.g.,by weight parameter calculator 209 (FIG. 2). The at least one weight, orboth of the weights, if both are calculated, are calculated in the samemanner as described above. It is only necessary to calculate one of theweights which can then act as the weight parameter, from which the otherweight can be determined in the transmitter using the relationshipdescribed above.

Thereafter, in step 407, either both weights or the determined weightparameter is supplied by the receiver of the forward link to thetransmitter of forward link, e.g., via the reverse channel. The weightis stored in weight supplier 105 (FIG. 1). If only one weight issupplied as a weigh parameter, the other weight is computed in weightsupplier 105 and then also stored therein.

In step 409, the input data stream is divided into M+1 substreams e.g.,by demultiplexer 101 (FIG. 1). Each of the first M data substreams isthen multiplied by weight α₁ in step 411 (FIG. 4). In other words, eachbit of each of each particular data stream is multiplied by α₁ toproduce M weighted data substreams. Additionally, the M+1^(th) datasubstream is multiplied by α₂ to produce the M+1^(th) weighted datasubstream.

In step 413 each of the first M weighted data substreams is combinedwith the M+1^(th) weighted data substream, e.g., by adders 109. Theprocess then exits in step 415.

FIG. 5 shows an another exemplary portion of a transmitter fordeveloping signals to transmit in a MIMO system having a transmitterwith M transmit antennas transmitting over a forward channel, such thateven in the face of some correlation the most open-loop capacity thatcan be achieved with a channel of that level of correlation is obtained,in accordance with the principles of the invention. Shown in FIG. 5 area) demultiplexers (demux) 501 and 503; b) weight supplier 505; c)antennas 507, including antennas 507-1 through 507-M; d) adders 509,including adders 509-1 through 509-M; e) multipliers 511-1 and 511-2;and f) radio frequency (RF) converters 515; including 515-1 through515-M.

Demultiplexer 501 takes a data stream as an input and supplies as anoutput two data substreams by supplying various bits from the input datastream to each of the data substreams. The first data substream issupplied by demultiplexer 501 to multiplier 511-1 while the second datasubstream is supplied to multiplier 511-2. Multiplier 511-1 multiplieseach value of the first substream by a first weight supplied by weightsupplier 505. Similarly, multiplier 511-2 multiplies each value of thesecond substream by a second weight supplied by weight supplier 505.

The first and second weights may be related to each other, and may bedeveloped by weight supplier 505 from a common weight parameter whichmay be derived from statistics of the forward channel, as will bedescribed in more detail hereinbelow. In one embodiment of theinvention, weight supplier 505 actually develops the weight values inresponse to information received via the reverse channel from thereceiver shown and described herein above in connection with FIG. 2. Inanother embodiment of the invention the weight values are developed inthe receiver, then supplied via the reverse channel to the transmitter,in which they are stored in weight supplier 505 until such time as theyare required.

Demultiplexer 503 takes the weighted data substream supplied as anoutput by multiplier 511-1 and supplies as an output M weighted datasubstreams by supplying various bits from the weighted data substream itreceived to each of the data M weighted substreams. Typically, each ofthe M weighted data substreams are of equal rate. Each of the M weighteddata substreams developed by demultiplexer 503 is supplied as an inputof a respective one of adders 509. Each of adders 509 also receives atits other input the weighted second substream which is supplied as anoutput by multiplier 511-2. Each of adders 509 combines the two weighteddata substreams input to it so as to produce a combined branch signal.Thus, M combined branch signal are produced, one by each of adders 509.Each of radio frequency (RF) converters 515 receives one of the Mcombined branch signals and develops therefrom radio frequency versionsof the M combined branch signals, which are then supplied to respectiveones of antennas 507 for transmission.

In another embodiment of the invention, for use with so-called “timedivision duplex” (TDD) systems, which share a single channel for boththe forward and reverse channels, the calculation of the correlationamong the channel components η may be performed at either end of thewireless link. This is because, since the forward and reverse channelsshare the same frequency channel, alternating between which is using thechannel at any one time, the channel statistics for the forward andreverse channels will be the same. Therefore, the receiver of thereverse channel will experience the same correlation among the channelcomponents η as the receiver of the forward channel, and so the receiverof the reverse link can measure the correlation among the channelcomponents η that was previously measured by the receiver of the forwardlink. Likewise, the receiver of the forward channel will experience thesame channel response as the receiver of the reverse channel, and so thereceiver of the forward link can determine the correlation among thechannel components η that were previously determined by the receiver ofthe reverse link. However, the SINR must still be computed only at thereceiver and relayed to the transmitter if necessary.

1. A receiver for use in a MIMO system, comprising: N receive antennas;N radio frequency (RF) converters, each RF converter downconverting asignal it receives from a respective associated antenna to an analogbaseband signal and converting said analog baseband signal to a digitalrepresentation; an estimator, responsive to said digitalrepresentations, for determining an estimate of the averagesignal-to-interference-and-noise ratio (SINR) for a forward channelbeing received by said receiver; an estimator for determining anestimate of a correlation among the channel components for a forwardchannel being received by said receiver; and a weight calculator forcalculating at least one weight for use by a transmitter of said forwardchannel to transmit data substreams to said receiver as a function ofsaid estimates of SINR end correlation among the channel components,said at least one weight being determined in said weight calculator bysolving at least one equation of the set consisting of $\begin{matrix}{\alpha_{1} = \sqrt{\frac{P_{T}}{M} - \alpha_{2}^{2}}} \\{\alpha_{2} = \sqrt{\frac{P_{T}\eta}{\rho\;{N( {1 - \eta} )}( {{M\;\eta} + 1 - \eta} )}}}\end{matrix}$ where α₁ and α₂ are said first and second weights,respectively, P_(T) is the total available transmit power ρ is anestimate of the average signal-to-interference-and-noise ratio (SINR),and η is the correlation among the channel components, M is the numberof transmit antennas, and N is the number of receiver antennas.